Messenger RNA-directed protein synthesis is catalyzed in all cells by a highly conserved[unreadable] ribonucleoprotein enzyme called the ribosome. Atomic resolution crystal structures of ribosomes, the first of[unreadable] which were reported 4 years ago, have had a revolutionary impact on our understanding of protein synthesis,[unreadable] but many interesting questions remain that can best be approached crystallographically. The research[unreadable] proposed for the next 5 years has two components. First, we intend to use the crystallographic and genetic[unreadable] tools already developed here for Haloarcula marismortui (Hma) to obtain new insights into ribosome[unreadable] structure and function. Second, crystals of ribosomes and ribosomal subunits will be prepared from new[unreadable] species so that questions about ribosome structure and function can be answered that cannot be addressed[unreadable] using any of the ribosome crystals now available.[unreadable] Specifically, the hypothesis that the well known differences in the properties of the peptidyl transferase[unreadable] centers of the ribosomes from different species are caused by interactions between the nucleotides in its[unreadable] conserved core with more remote, non-conserved nucleotides will be tested in the Hma large ribosomal[unreadable] subunit using a combination of genetics and crystallography. The same tools will also be used to determine[unreadable] how the peptidyl transferase center responds conformationally to the mutation of highly conserved bases in[unreadable] the peptidyl transferase center. In addition, a series of experiments will be carried out the objective of which[unreadable] is to prepare large subunit that have nascent peptides in their exit tunnels, and solve their structures[unreadable] crystallographically. The final goal is the preparation of crystals of eukaryotic ribosomes that diffract to[unreadable] atomic resolution, and the determination of their structures.